The Little Manual of API Design (PDF)

This manual gathers together the key insights into API design that were discovered through many years of software development on the Qt application development framework at Trolltech (now part of Nokia). When designing and implementing a library, you should also keep other factors in mind, such as ef´Čüciency and ease of implementation, in addition to pure API considerations. And although the focus is on public APIs, there is no harm in applying the principles described here when writing application code or internal library code.

A useful reference.